Allometric equations for Sequoia sempervirens in forests of different ages.

Highlights • New allometric equations for Sequoia sempervirens in second-growth forest. • Evaluation of bias in allometric equations derived from forests of different ages. • Complete structural analysis of 159-year-old forest in Fritz Wonder Plot. • Estimating aboveground biomass and carbon stock in mature and old-growth forests. • Comparison of long-term tree performance and canopy structure in alluvial forests. Abstract Mature second-growth forests dominated by Sequoia sempervirens occupy only two percent of the species' current distribution yet represent an important benchmark for restoration management. Here we develop new allometric equations for these forests based on 44 trees 23–84 m tall, which can be used to estimate leaf, bark, cambium, sapwood, and heartwood quantities, including biomass and carbon content, from ground-based measurements of trunk diameter, height, and crown size. New equations complement those already available for S. sempervirens in young and old-growth forests and should be applied judiciously. We use fixed area plots in a 159-year-old second-growth forest (0.5 ha) and a primary forest with trees >1100 years old (1.0 ha) to compare three sets of equations. Whereas equations from young forests underestimate biomass in older forests, new equations underestimate trunk and overestimate branch quantities in the old-growth forest, and equations from old-growth forests do the opposite in the mature second-growth forest. Despite the great age difference, canopy structure of these low-elevation, alluvial forests is similar with S. sempervirens stratified into dominant and subordinate crown classes towering above a diminutive understory. Dendrochronological reconstruction of heights and wood radii for nine individuals reveal that both forests have trees producing >300 kg yr⁻¹ with tall trees in the old-growth forest producing up to 800 kg yr⁻¹ in recent decades. Sequoia sempervirens represents nearly 100% of biomass and 83 to –92% of leaf area in the second-growth (total LAI = 15.0) and old-growth (total LAI = 19.3) forests, respectively. The best allometric equations estimate that the second-growth forest holds 1667 Mg ha⁻¹ of aboveground biomass (837 Mg C ha⁻¹), 56% of which is heartwood, whereas the old-growth forest holds 4595 Mg ha⁻¹ of aboveground biomass (2332 Mg C ha⁻¹), 76% of which is heartwood. Restoration management has potential to harness the carbon sequestration potential of S. sempervirens by promoting heartwood accumulation through silviculture. [ABSTRACT FROM AUTHOR]